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DEFECT CONTROL IN VACUUM BAG ONLY PROCESSING OF COMPOSITE PREPREGS
by
Lessa Kay Grunenfelder
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(MATERIALS SCIENCE)
December 2012
Copyright 2012 Lessa Kay Grunenfelder

Composite parts for commercial aircraft are traditionally manufactured using high-pressure autoclave processing of prepregs (carbon fiber pre-impregnated with epoxy resin). In recent decades, however, the use of composite parts for aircraft has increased, and aircraft markets have grown, creating pressure to increase production rates. To meet the growing demand for composite aircraft parts and to allow for the production of large composite components (i.e. wings and fuselage) alternative processing methods will be required. There are several drawbacks to autoclave processing, including a large capital investment, long cycle time, high cost of the nitrogen gas used to pressurize the vessel, size limitations, and poor energy efficiency. New out-of-autoclave processing methods have been developed to address these issues. One such method is vacuum-bag-only (VBO) processing of prepregs, a technique which uses atmospheric pressure alone to consolidate parts. ❧ VBO processing presents a potential solution for the manufacture of larger parts at faster rates using conventional layup and placement tools. However, before VBO methods can be used on primary structure, the quality of VBO processed parts must be shown to be equivalent to that of autoclave cured parts. The elimination of high external pressures during the cure cycle removes safeguards in the manufacturing process, resulting in the need for strict protocols in the layup and cure of VBO parts. To assess the feasibility of VBO processing for aerospace components a systematic study of the effect of process parameters on the quality of VBO parts is essential. Specifically, the mechanisms of void formation and growth in prepreg-processed carbon fiber composites are not well understood. The purpose of this work is to examine the potential causes of voids, to develop a complete understanding of the mechanisms of void formation. This knowledge will aid in the production of higher quality parts, and help to determine the feasibility of low-pressure VBO processing for large-scale structural components. ❧ As a starting point, carbon fiber/epoxy test laminates were manufactured using vacuum bag only methods as well as traditional autoclave cure cycles. Cured laminates were tested using aerospace qualification standards. Tests were performed on dry laminates as well as laminates that had been hot/wet conditioned. Mechanical properties were shown to be equivalent in vacuum bag only and autoclave processed laminates, and values for all test panels and test conditions exceeded the required level for structural aerospace applications. ❧ Cure cycle optimization was carried out to further improve the properties of out-of-autoclave processed parts. Variations in hold time and temperature were investigated for the first temperature dwell of the cure cycle. Several test panels were fabricated with a range of processing times and temperatures. Cured laminates were characterized for compaction (thickness), void content, and surface finish. Resin rheological properties were also examined. Based on experimental results, an optimized cure cycle was developed for the material system studied. ❧ The mechanisms of impregnation and compaction in vacuum-bag-only prepreg materials were investigated to determine the influence of fiber architecture on flow and void removal in out-of-autoclave processing. Material microstructure and laminate thickness were tracked as a function of cure time and temperature for a unidirectional material and a 5-harness satin woven prepreg featuring the same resin system. Isothermal resin flow was analyzed to determine the activation energy for resin impregnation in each prepreg, a quantitative measure of the influence of fabric type on flow behavior. Impregnation in the unidirectional material occurred early in the cure process, followed by additional ply compaction. In contrast, impregnation and compaction occurred on a longer time scale for the woven fabric. The impregnation, compaction, and air removal mechanisms observed for unidirectional and woven fabrics were different, indicating that cure cycles must be tailored to fiber architectures in prepregs, and possibly to part geometry. ❧ Void formation as a function of resin moisture content was studied to better understand and control process defects in composite parts made from prepreg. Uncured prepreg was conditioned at 70, 80 and 90% relative humidity and at 35°C. Conditioned prepreg was laid up into 16-ply laminates and cured using vacuum bag only processing, as well as partial vacuum and autoclave processing. Moisture uptake in the resin was measured using coulometric Fischer titration. Void content was measured by image analysis of polished sections of cured laminates. Void content increased substantially with increasing moisture content in vacuum bag only processed samples, and a strong pressure dependence was noted. Under autoclave cure conditions, void-free parts were produced even at high moisture levels. Experimental results were compared with trends predicted using a diffusion-based analytical model. ❧ Changes in vacuum bag only prepreg properties were tracked as a function of room temperature aging time (out-time). A modulated differential scanning calorimetry method was used to characterize the prepreg, and changes in prepreg tack levels were examined using an energy of separation technique. Laminates were cured from prepreg at various levels of aging using both traditional autoclave processing and low-pressure VBO techniques. Cured laminates were examined using ultrasound scanning, mechanical properties were tested, and laminates were sectioned for investigation of microstructure. Laminate quality as a function of out-time was examined in terms of prepreg properties and manufacturing technique. ❧ The results of the preceding study indicated that a need exists for an accurate and convenient method to monitor the extent of prepreg aging as a function of out-time. For this reason, a method to track prepreg age was developed, involving measurement of changes in glass transition temperature as a function of room-temperature aging time. Samples from three out-of-autoclave prepreg systems were aged in ambient conditions and tested periodically using modulated differential scanning calorimetry. A linear increase in glass transition temperature with prepreg age was noted. ❧ In addition to monolithic parts and flat laminates, prepregs are used for face sheets of sandwich structures, where they are bonded to low-density cores of honeycomb or foam. To reduce processing time and cost, co-cure of composite face sheets and sandwich structure adhesives is desirable. VBO processing is attractive for this application, as it allows for the use of lighter and less costly core materials, eliminating the risk of core crush that can result from high autoclave pressures. Under vacuum, however, sandwich structure adhesives often foam as gas species evolve from solution. The first step in eliminating adhesive foaming is to identify the volatiles evolving from adhesives during cure. An on-line coupled thermogravimetric analyzer-Fourier transform infrared spectrometer (TGA-FTIR) technique was employed to identify volatile components evolving during the cure of two polyimide film adhesives. ❧ Overall, the work presented here provides an analysis of the influence of various process parameters on voids, leading to an improved understanding of the mechanisms of void formation in VBO processed parts. The results of this investigation shed light on the cure protocols required for the production of high quality composite parts in the absence of autoclave pressures.

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DEFECT CONTROL IN VACUUM BAG ONLY PROCESSING OF COMPOSITE PREPREGS
by
Lessa Kay Grunenfelder
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(MATERIALS SCIENCE)
December 2012
Copyright 2012 Lessa Kay Grunenfelder